Updating default theme

Description

Default theme is the first thing theme designers will look at during designing themes, and trying to follow its way, so keep it updated. If some feature developed for themes and not enabled in default theme, no other theme will enable it.

Features not included in default theme:

RTL support:

Version 2.1 comes with support for RTL languages enabled in admin and available for themes, unforuontly default theme don't enable it.

Custom Header support:

Default theme contain custom header in it, version 2.1 implemented it in WP core, but default theme still use its custom code.

L10N support:

Default theme don't support L10N, so there are no L10N themes. This issue applied to plugins too (See #2509).